Mental retardation is a generalized disorder appearing before adulthood, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors. It has historically been defined as an Intelligence Quotient score under 70. Once focused almost entirely on cognition, the definition now includes both a component relating to mental functioning and one relating to individuals' functional skills in their environment. As a result, a person with a below-average intelligence quotient (BAIQ) may not be considered mentally retarded. Syndromic mental retardation is intellectual deficits associated with other medical and behavioral signs and symptoms. Non-syndromic mental retardation refers to intellectual deficits that appear without other abnormalities.
The terms used to describe this condition are subject to a process called the euphemism treadmill. This means that whatever term is chosen for this condition, it eventually becomes perceived as an insult. The terms mental retardation and mentally retarded were invented in the middle of the 20th century to replace the previous set of terms, which were deemed to have become offensive. By the end of the 20th century, these terms themselves have come to be widely seen as disparaging and politically incorrect and in need of replacement. The term intellectual disability or intellectually challenged is now preferred by most advocates in most English-speaking countries. Clinically, however, mental retardation is a subtype of intellectual disability, which is a broader concept and includes intellectual deficits that are too mild to properly qualify as mental retardation, too specific (as in specific learning disability), or acquired later in life, through acquired brain injuries or neurodegenerative diseases like dementia. Intellectual disabilities may appear at any age. Developmental disability is any disability that is due to problems with growth and development. This term encompasses many congenital medical conditions that have no mental or intellectual components, although it, too, is sometimes used as a euphemism for MR. Because of its specificity and lack of confusion with other conditions, mental retardation is still the term most widely used and recommended for use in professional medical settings, such as formal scientific research and health insurance paperwork.